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Chapter 3

Chapter 3. Input, Variables, Constants, And Calculations. 3.1 Introduction. Chapter 3 Topics. This chapter covers the use of text boxes to gather input from users It also discusses the use of variables named constants intrinsic functions mathematical calculations.

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Chapter 3

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  1. Chapter 3 Input, Variables, Constants, And Calculations

  2. 3.1Introduction

  3. Chapter 3 Topics • This chapter covers the use of text boxes to gather input from users • It also discusses the use of • variables • named constants • intrinsic functions • mathematical calculations

  4. 3.2Gathering Text Input In This Section, We Use the Textbox Control to Gather Input That the User Has Typed on the Keyboard

  5. Placing Text into a Label, I • We have done this already:lblSet.Text = "Place this text in a TextBox" • The lblSet.Text is in the form: Object.Property

  6. Placing Text into a Label, II • The text can come from a textBox where the user has typed in input:lblSet.Text = txtInput.Text • Notice two use of the form: Object.Property

  7. Clearing a Text Box, I • This can be done with an assignment:txtInput.Text = "" • Two adjacent quote marks yields a null string • So this statement replaces whatever text that may have been in txtInput with "nothing" -- a string with no characters in it

  8. Clearing a Text Box, II • This can be done with a method:txtInput.Clear() • Clear is called a Method • Methods do actions -- here clearing the text • The syntax is similar to that of referring to a Property: Object.Method

  9. String Concatenation, I • In our code we will often need to combine two or more strings into a longer one • This operation is called "Concatenation" • Concatenation is signaled with the operator '&' much in the same way that addition is signaled by the operator '+'

  10. String Concatenation, II • Say our user has entered their name into txtUserName, a TextBox • In label lblGreeting we want to say, Hello • Simply:lblGreeting.Text = "Hello " & txtUserName.Text • Put "Hello" on the front of the user's name and place the result into lblGreeting

  11. The Focus Method, I • For a control to have the focus means that it is ready to receive the user's input • In a running form, one of the controls always has the focus • The control with the focus may be set by program control using the Focus Method:txtUserName.Focus()

  12. The Focus Method, II • You can tell which control has focus by its characteristics: • When a TextBox has focus, it will have a blinking cursor or the text inside of the box is highlighted • When a button, radio button, or a check box has focus, it will have a thin dotted line around the control

  13. Controlling a Form’s Tab Orderwith the TabIndex Property • Stepping the focus from one control to another can be done using the Tab Key • This order is set for a control relative to others by the value of the TabIndex Property • With each Tab Key hit, the focus will step to the control with the next highest value of the TabIndex Property

  14. Assigning Keyboard Access Keysto Buttons • Imagine your form has a button with the text "Save" on it • And you wish to allow the user to be able to hit Alt-S to activate that button • Simply change the button text to "&Save" • The '&' tells Visual Basic .NET to use Alt-S as an access key

  15. '&' is a Special Characterin Button Labels • Note that the '&' in "&Save" does not display on the button • It simply establishes the Alt Key access • In order to actually display an '&' on a button, one must enter it as "&&" (then one will appear and will not cause an Alt Key access to be established)

  16. Using Access Keys with Labels, I • Want to establish an access key for a TextBox? • The previous technique will not work because the text of a TextBox is normally a changing value • However, there is a way to accomplish the same effect

  17. Using Access Keys with Labels, II • For a Label that immediately precedes a TextBox • Assign that Label an access key (labels do not normally have access keys) • Set the UseMnemonic Property to True • When the user activates the Label's access key, the following TextBox will receive the focus

  18. Setting the Accept Button • The Accept Button is the one that implicitly will be activated if the user hits the Enter Key • The AcceptButton Property designates which button on the form is to behave in this manner

  19. Setting the Cancel Button • The Cancel Button is the one that implicitly will be activated if the user hits the Escape Key • The CancelButton Property designates which button on the form is to behave in this manner

  20. 3.3Variables An Application Uses Variables to Hold Information So It May Be Manipulated, Used to Manipulate Other Information, or Remembered for Later Use

  21. Why Have Variables? • A variable is a storage location in the computer’s memory, used for holding information while the program is running • The information that is stored in a variable may change, hence the name “variable”

  22. What Can You Do With Variables? • Copy and store values entered by the user, so they may be manipulated • Perform arithmetic on values • Test values to determine that they meet some criterion • Temporarily hold and manipulate the value of a control property • Remember information for later use in the program

  23. How to Think About Variables • You the programmer make up a name for the variable • Visual Basic .NET associates that name with a location in the computer's RAM • The value currently associated with the variable is stored in that memory location

  24. Setting the Value of a Variable • An assignment statement is used to set the (new) value of a variable, as in:length = 112 greeting = "Good Morning " & txtName.Text

  25. Variable Declarations • A variable declaration is a statement that causes Visual Basic .NET to create a variable in memory • As inDim length As Integer

  26. Declaration Syntax • The official syntax is Dim VariableName As DataTypewhere • Dim (stands for Dimension) is a keyword • VariableName is the name to be used • As is a keyword • DataType is the type of the variable and will be one of many possible keywords

  27. Boolean Byte Char Date Decimal Double Integer Long Object Short Single String Visual Basic .NET Data Types

  28. Variable Naming Rules • The first character of a variable name must be a character or an underscore • Subsequent characters may be either of those plus the numeric digits • Thus variable names cannot contain spaces or periods (or many other kinds of characters) • Variable names must not be keywords

  29. Variable Naming Conventions • Each variable name should describe its use, e.g., itemsOrdered • When multiple words are used in a name, capitalize the initials, except for the first one (again, itemsOrdered) • As noted earlier, control names should have a specific prefix, e.g. btn for Button controls

  30. Auto List Feature • As you are entering your Visual Basic .NET program, VB will often aid you by offering a list of choices for that could be entered next • Right after you type "As" in a variable declaration, Visual Basic .NET will offer you a list of all of the established data types • Either choose one or keep typing

  31. Variable Default Values • When a variable is first created in memory, Visual Basic .NET assigns it a default value • numeric types are given a value of zero • strings are given a value of Nothing • dates default to 12:00:00 AM January 1,1

  32. Initialization of Variablesvia the Declaration • It is preferable to establish your program's own initial value for variables that will not otherwise be given values before they are used • In the declaration, simply append " = value"Dim length As Integer = 112

  33. Scope of a Variable, I • A variable’s scope is the part of the program where the variable is visible and may be accessed by programming statements

  34. Scope of a Variable, II • The scope of a variable begins where it is declared • And extends to the end of the procedure in which it appears • This kind of variable is called local • A local variable is declared inside a procedure • The variable is not visible outside of the procedure and its name cannot be declared again within the same procedure

  35. Lifetime of a Variable • The storage for a variable is created upon each use of the procedure • The storage for a variable is destroyed as soon as the procedure finishes executing

  36. Setting a Specific Date • This can be done a number of ways:startDate = #12/3/2002 1:00:00 AM#startDate = System.Convert.ToDateTime( "12/3/2002 1:00:00 AM")

  37. Setting the Current Date/Time • A series of keywords yields the date and time or just one or the other: • Now startTime = Now • TimeOfDay startTime = TimeOfDay • Today startTime = Today

  38. The Val Function, I • Suppose you wish to use text input as a number:number = txtInput.Text • This will work without a run time error as long as txtInput.Text is the text equivalent of a numerical value (like "45") • If it is not, there will be a run time error

  39. The Val Function, II • The Val function is more lenient on conversions from text to numeric values • If the initial characters form a numeric value, it will return that • Otherwise, it will return a value of zero

  40. The Val Function, III • Argument Val(Argument) • "34.90" 34.9 • "86abc" 86 • "$24.95" 0 • "3,789" 3 • "" 0 • "x29" 0 • "47%" 47 • "Geraldine" 0

  41. ToString Method • This is a Method that will convert any variable to a string, as inDim number As Integer = 123 lblNumber.Text = number.ToString

  42. Option Strict On • Placed at the very top of the code window this will prevent Visual Basic .NET from performing implicit data type conversion • The code must perform all conversions using Val or ToString (or another similar conversion procedure such as CInt or Integer.Parse)

  43. 3.4Performing Calculationsand Working With Numbers Visual Basic .NET Provides Several Operators for Performing Mathematical Operations You May Also Use Parentheses to Group Operations and Build More Complex Mathematical Statements

  44. The Arithmetic Operators, I • Visual Basic .NET provides operators for the common arithmetic operations: • Addition + • Subtraction - • Multiplication * • Division / • Exponentiation ^

  45. The Arithmetic Operators, II • Examples of use: • total = price + tax • area = length * width • average = total / items • salePrice = retail / 2 • cubeArea = side ^ 3

  46. Special Integer Division Operator • The backslash (\) is used as an integer division operator • The result is always an integer, created by doing the division and then discarding any remainder • Any floating-point operand is first rounded to the nearest integer

  47. Special Modulo (MOD) Operator • This operator follows the same basic rules as the backslash operator, but yields the remainder after the division • \ operator yields an integer result of division • MOD operator yields the integer remainder (after division using the \ operator)

  48. Arithmetic Operator Precedence, I • Which operations are done first -- precedence tells us -- highest to lowest: • Exponentiation (^) • Multiplicative (* and /) • Integer Division (\) • Modulus (MOD) • Additive (+ and -)

  49. Arithmetic Operator Precedence, II • When two operators with the same precedence share an operand, the operator on the left works first, then the operator on the right

  50. Arithmetic Operator Precedence, III • Grouping with parentheses () forces the expression within those parentheses to be evaluated before others • Roughly speaking, the order of evaluation in Visual Basic .NET is similar to that used in algebra (parenthesized expressions first, then exponentiation, then multiplicative operators, then the additive operators)

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